Cathode follower photoelectric amplifier circuit



1952 R. w. BORDEWIECK ETAL 2,582,676

CATHODE FOLLOWER PHOTOELECTRIC AMPLIFIER CIRCUIT Filed Aug. 6, 1949 INVENTORJ' HMAmsw/f 70. f ce Patented Jan. 15, 1952 CATHODE F OLLOWER PHOTOELECTRIC AMPLIFIER CIRCUIT Robert W. Bordewieck, South'boro, James 0. Roebuck and Neal M. Sleeper, Worcester, Mass, alsignors to Moore Electronic Laboratories, Incorporated, Worcester, Mass., a. corporation of Massachusetts 7 Application August a, 1949, Serial No. 108,977

' 3Claims.

l The principal object of this invention is to provide an improved circuit for use in detecting and amplifying extremely low intensity light radiations, both visible and invisible.

Whereas, most investigators have found that so-called phototube dark current and drift in phototube sensitivity, make it impossible to detect and amplify extremely low levels of illumination without first either chopping the light sisnal by means of a motor or vibrator driven signal interrupter, or by the use of a modulated light source, our system eliminates the need for any of these. By employing special guard circuits, special types of amplifying and phototubes, and a specially balanced amplifier circuit with a special relay as a detector and a lens to increase the light signal on the phototube, we have been able to detect light intensities easily, which hitherto required special delicate precision laboratory equipment. While our circuit and equipment require special components, these components are readily available.

Other objects and advantages of the invention will appear hereinafter.

Reference is to be had to the accompanying drawings, in which Fig. l is a diagram of a circuit for the detection and measurement of low light intensities;

Fig. 2 is a diagram of an alternative relay detector circuit; and

Fig. 3 is a further modification thereof.

Referring to Fig. 1, our circuit for detection and measurement of low light intensities comprises a source of electrical energy at l connected as by lines II and I2 to a high voltage rectifier l3. If the source of electrical energy used is a battery, at l0, it is necessary to use a vibrator and transformer at ['3 to produce high voltage A. C. which is then rectified. If a low voltage A. C. source is used at III, a transformer is needed at l3 to increase this voltage before it is rectified; if a low voltage D. C. battery is used at 10, a dynamotor may be used at l3 to produce high voltage D. C. In any case, the negative connection of rectifier I3 is grounded as by line l4 and the high voltage terminal I5 is connected as by resistor It to point 11. A voltage regulator tube 20 is connected from point I! to ground by line HA. Resistor It serves as a ballast resistor for tube 20.

The pick-up circuit consists of a protective resistor 3| connected by lines l9 and Hi to point I1 and by line 38 to the anode 32 of a phototube 34. The cathode 33 of phototube 34 is connected as by ground shielded line 31 to the grid 44 of a tube 40. The envelope shield 36 of phototube 34 is connected by line 35 to ground. Grid 44 is connected through resistor 42 and condenser 4| to ground. The plates 45 and 55 of tubes 40 and 50 respectively are connected by lines 46 and 56 respectively to a line 23 and thence to the junction point on the voltage divider consisting of resistors 2| and 22. Resistor 2| connects to line l8 and thence to point ll. Resistor 22 connects to ground. Grid 54 of tube 50 is connected as by resistor 52 and condenser 5i to ground. Heaters 41 and 51 of tubes 46 and 56 respectively, are connected at one end to ground and at the other end by means of lines 48, 58, and 25 to the source of electrical energy Ii]. Cathodes 43 and 53 are connected as by ground shielded lines 49 and 59 to the resistors 62 and 12, connected in the grid circuits of tubes 60 and 10. Resistor 12 connects directly from grid 14 of tube 10 to ground. Resistor 62 is a potentiometer, the moving contact 62A thereof being connected by line 6| to grid 64 of tube 60. The heaters 61 and 11 of tubes 60 and 10 respectively are connected by one end to ground and by the other end through lines 68 and I8 and lines 26 and 25 to the source of electrical energy l0.

Cathodes 63 and 13 are connected as by lines 69 and '19 to resistor 86 and thence to ground. Plates and 15 are connected as by lines 8! and 82 to the detecting relay network provided and thence by line 21 to the anode of voltage reg- ;ulator tube 20.

Three options are provided for this detecting network. In Fig. 1, the detecting network option chosen is a differential relay with two coils 83 and 84, with a common centrally pivoted armature 90, there being a spring 86 adjusted to hold the armature against contact 85, with no current through the coils 83 and. 84. If the current through the coil 84 is more than that through coil 83, its force aids the spring 86 and continues to hold armature 90 on contact 85. If the current through coil 83 is greater than that of coil 84, armature 90 is pulled against the force of the spring 86 and armature 90 closes against contact 9|. Effectively, the amount of illumination striking the phototube controls the single pole double throw switch, consisting of the armature 90 and contacts and 9|, thus enabling the signal to open or to close a circuit between lines 8? and 89 or 81 and 8B.

The operation of the circuit for Fig. l is as follows: With no light striking the phototube 34, no current will flow through that tube and effectively. grids 44 and 54 are connected to ground through equal resistors s2 and 52 respectively. The plates of the cathode foilower tubes at and til are connected to a low voltage tap (6 to 10 volts D. C.) at the junction point between resistors 2i and 22. The cathodes 43 and 53 of the cathode follower tubes and respectively are connected through resistors 62 and "i2 respectively to ground and with no signal applied, suflicient voltage will be developed across resistors E2 and 2 to self-bias the tube 40 and with a slight negative voltage (approximate- 1 volt). This voltage developed from cathode to ground of the cathode follower tubes 40 and til is applied to the grids of the alhplifier stage tubes 5% and it is applied to the grids of the amplifier stage tubes and ll] as a positive voltage. This positive voltage from grid to ground causes an appreciable amount of plate current in both tubes 66 and 70, which plate currents running through common cathode resistor Bil, proceed to develop s'ufiicient self-bias to limit those plate currents and thereby set thequiescent operating plate currents of tubes 6t and 70 lt'is desirable that the current through tube tube slightly less than that of tube 30 so that armature 90 will be held by the spring, as well by the means of the relay coils 83 and 84 onto contact 85. This is accomplished by decreasing slightly the positive voltage applied to grid 64 by means of the potentiometer slides 62A. When light is focused through lens 24 onto phototube cathode '33, current passes through the phototube and resistor 42 to ground, put

creasing' the positive voltage applied to grid 64 and thereby'increa's'es theplate current through coil 83. This increased current through coil 63 causes armature 90 to be. pulled against the force of spring 86 from contact onto con-' tact 9i, thereby'switching the circuits upon the applicationof a light signal. If the plate current through tube 69 increases, the current through resistorflw increases, which increases theself bias on both tubes 60 and 10 somewhat. 'Ihis increased bias is always less than the increased voltage applied from contact 62A to ground, but it has theeffec't of decreasing the current through tube 10.

That is, taking the circuit from cathode 63 through resistor 80, through resistor 82 to grid Gian increase in light signals causes the voltage to become'more positive in spite'of the increased curr'ehtthrough resistor 80; whereas taking the circuit from cathode 73 through resistor 80 and resistor'lZto grid 1 4, grid 14 becomes more negative because of the increased voltage drop across resistor 80. The voltage across resistor 72 remains constant with or without a light signal as it is "not connected :to the phototube. Thus it lsse'en that an increase in light signal increases ly by the cathode follower but its impedance is reduced from, say megohms down to megohm, enabling the use of conventional tubes the amplifiers 60 and I0. Lens 24 serves to also increase the sensitivity of the circuit and provides desirable directional properties. If lens :34 is colored, or if filter 28 is inserted behind 24, the circuit can be made to respond only to certain light frequencies or can be made to not respond to certain ll'ght'freduencies.

Extremely stable operation is provided by using voltage regulator tube2ll to control the voltage applied to 'the 'ainplifiers. A special limiting action is provided which prevents the signal output from th "tho'de follower ever exceeding 6 to 8 volts by connecting the plates of the cathode follower to .avoltage divider providing 6 to 8 voltsfShould a voltage of over 6 volts be applied'to the grid acrossthe resistor 42 the grid would be more positive than the plate, and due to the grid-current that would then flow in the high impedance tube circuit, a riSe of greater voltage over that of the plate voltage is almost impossible. This feature protects the tube and circuit elements.

It has been found that the so-called dark current of a "gas-phototube operating at 75 volts is due almostexclusively to surface leakage and leakage through'its'base', so a special guard "circuit was devised to control any leakage between the anode and cathode. having the anode and'ca'thode insulated by glass to give a long surface leakage path is used, and a conducting surface .between the terminals connected to ground thereby causing any leakage across the tube surface to go, not to the cathode,- where it would act as a signal, but to the ground. In the base for the phototube, a similar conducting strip is employed to prevent leakage between the socket terminals of the phototube; The leadconne'cting the cathode oi the phototube to the grid of the cathode follower is shielded by a'grounded shield, thus, any stray pick-up is avoided in this line and leakage from high voltage points to the grid through this wire is avoided. The shield'is not always necessary for proper operation depending upon the length of line 31.

Should it be desired to slow 'downthe operation of the circuit and put a time lag in the operation, this can readily be 'done by'connecting a condenser 4| in parallel with resistor 42. This cond'enser'resistor combination has an unpredictable time constant in combination with the phototube 34. If a strong light signal suddenly strikes the phototube, condenser 41 will be charged quickly and'its'delay action will be fast. If the light signal 'issuddenl'y removed from the phototube, condenser 4! will discharge rather rapidly through resistor 42, but if th'e light signal is decreased slightly, the condenser will provide a time 'delay, s1owing upth'e action of'the circuit for either a slight increase or a 'sligl'lt decrease in light. Condensers 4| and 5! maybe A special phototubethe relay is actuated it is pulled from contact I to contact IOI, line 81A connects toarmature 99, line 88A connects to contact IN, and line 89A connects to contact I00. Operation of this circuit is similar to that of Fig. 1. With no signal the current through resistor 92 is less than that of resistor 93. Therefore point 96 will be at a higher potential than point 91 which would tend to cause current to flow through coil 04 and rectifier 95; however, rectifier 95 does not allow current to flow in this direction. When the light signal is increased, the current through resistor 92 causes a greater IR. drop across it and as explained in Fig. l, the current through resistor 93 will be decreased somewhat. This causes the voltage at point 01 to be higher than that at point 96, and current flows from point .91 through rectifier 05, coil 94 to point 96, actuating the relay and switching armature 09 from contact I00 to contact Ill. The rectifier is inserted so that the relay will not switch at zero light signal, due to current in the opposite direction. Rectifier 95 could be omitted if tubes were tested for balance and adjustment made so that the currents through 92 and 93 were equal with no light signal instead of the current through 93 being larger than that through 92 with no light signal. Operation of all the other parts of the circuits would be identically the same as that in Fig.1.

Referring to Fig. 3, a third alternative relay detector network is shown. In this network a polarized relay is provided which has a permanent magnet attached to the armature. Line 213 connects to line 21, line MB to 8|, and line 823 to 82. Resistor I02 is thus inserted in the plate circuit of tube 60 and resistor I03 is inserted in the plate circuit of tube 10, polarized relay coil I06 is connected between points I04 and I05, and magnet end II3 becomes a south pole attracting the north pole II! of the armature I0I, thus switching the armature I01 from contact III to contact IIO. Line 888 connects contact IIO, line 813 connects to armature I01, and line 89B connects to contact III. With no light signal, current fiows from point I05 to point I04 which creates a north pole at end II3 repelling the north pole of the armature II2 hence, not switching it. When light strikes the phototube, the voltage at point I05 drops, and current flows from point I04 through coil I06 to point I05 creating a south pole at end II3 which attracts the armatures' north pole at II2, switching armature from contacts I I I to contact I I0 with the increase in light.

By proper choice of the resistor in these circuits, an extremely stable circuit with high transconductance can be produced. The cathode follower circuit cathode to ground resistors should be high to decrease the effect of grid emission and increase the stability; however, these circuit resistors can be 1/1000 as large as the grid to ground resistance by connecting the plates of the cathode follower to a low voltage source ionization is practically eliminated. If resistor 29 is inserted in series with the heaters of the cathode follower tubes the grid emission is reduced. The voltage regulator tube provides a stable high voltage reference, both for the phototube and for the amplifier. If voltage regulator tube provides higher than 80 volts, a voltage divider network consisting of resistor H5 and III; may be used to provide the phototube with its required voltage. Resistor IIO may be replaced by a miniature neon tube. It is highly desirable to mount the cathode follower directly with the phototube in a sealed pick-up unit. A ballast tube may be used in place of dropping resistor 29 to provide a constant heater current. Lines 49 and 50 should be shielded if they are of any length to prevent pick-up being fed into the bridge circuit. Tubes 00 and I0 may be in the same envelope as a twin triode, or tubes 60 and 10 may each be twin triodes or pentodes to secure a high transconductance to make possible the use of a more rugged, cheaper relay.

In all three of the options for the output circuit, the hot side of the source of electrical energy is connected to the armature of the relay and the high beam filaments of an automobile may be connected to the normally closed contacts as by line 89 'while the low beam filaments may be connected to the normally open contacts as by line 08. Therefore a light signal can be made to switch the source of electrical energy from the high beams to the low beams and when the light is removed the power will be returned to the high beam filaments.

We are aware that prior to our invention.

automatic headlight control devices have been developed which have not proved practical because of the problems of instability and variation in tube and circuit elements. Because of the balanced structure of our circuit and the high grid resistors which we use in our cathode follower circuit, the sensitivity of our circuit is increased 10 to times over that of any other circuit available while still retaining a high degree of stability.

We, therefore, do not claim exclusive invention of an automatic headlight control circuit but rather improvements on the circuit.

We therefore claim:

1. A light actuated control circuit comprising 1 a source of electrical energy, a phototube circuit,

a D. C. amplifier comprising a bridge circuit, a cathode follower circuit interposed between said phototube circuit and said D. C. amplifier, a relay driven by said D. C. amplifier, said phototube circuit comprising a phototube and a resistor in series having the phototube anode connected to the positive terminal of said source of electrical energy and having the phototube cathode connected through said resistor to the negative terminal of said source of electrical energy, said cathode follower circuit comprising a vacuum tube having its grid connected to the cathode of said phototube, the vacuum tube plate connected to the positive terminal of said source of electrical energy, and having the cathode thereof connected througha suitable load resistor to the negative terminal of said source of electrical energy, said bridge circuit comprising two vacuum tubes having their plates connected through suitable plate load impedances to the positive terminal of said source of electrical energy, and having their cathode connected through a common resistor to the negative terminal of the said source of electrical energy, while having the control grid of one of them connected to the cathode of said cathode follower, a second cathode follower circuit similar to the first said cathode follower circuit but having connection from its cathode to the control grid of the other said vacuum tube in said bridge circuit, said second cathode follower circuit being effective to balance out any component of voltage in the output of said D. C. amplifier due to the quiescent output voltage of the first said cathode follower circuit and being efiective to maintain the improved 7 stability realized by bri'dging thesald D. C. amphfier.

2. The circuit/of claim 1wherein'the said relay is connected between the plates of the vacuum tubes of the said'br'idge circuit and wherein the circuit includes means connected in series with the relay @011 such that the relay may be energizedonly by current flowing in but one :g-ive'n direction through the said coil.

3. The circuit of claim 1 wherein the said relay is a differential relay having two "coils, said relay-being energized according to the differen-tial between the currents flowing in 'the twos'aidcoils; and each of the-two said'coils-functioning :as a means for connecting one of the platesof the vacuum tubes of 'sald 'D. Ceamplifi'er to theposi tive terminal of said source of electrical eneigy.

ROBERT W. BORDEWIECK. JAMES O. ROEBUGK. NEAL SLEEPER.

CTHER REF E'RENCE'S l ne R-adio Amateur s Handbook, 1945 edit, page-180. 

